Knocking noise partially solved - PSS Shaft Seal

Steve Mitchell 2 min read
Knocking noise partially solved - PSS Shaft Seal

For a while I’ve had a knocking noise when at low idle after the engine repower.  After a lot of trips to the bilge trying to figure out what’s going on, I found that the PSS Dripless Shaft Seal was dripping.  This has been ongoing now for 6 weeks or so.  The seal is a stainless steel disc attached to the propeller shaft near the transmission, and a matching set of bellows and a carbon disc that pushes up against the steel disc.  The stainless steel disc rotates with the prop shaft, while the carbon disk stays still, and is pressed against the steel disc.  Water lubricates the carbon disc so that it doesn’t heat up, and the whole thing is supposed to provide a dripless solution that is better than a stuffing box because of low maintenance.

During the engine repower, this was not replaced.  The shop that did the work realized afterwards that it probably would have been a good idea to do that since we replaced everything else around it.  Nevertheless, they have been extremely helpful in solving the issue.

At first, I tried adjusting the tension of the bellows that the carbon disc is attached to – by increasing the pressure, the drip should stop.  In the process, I found that the knocking noise at idle was coming from the carbon disc smacking the stainless steel disc.  Through a lot of trial and error, I did minimize the drip.  It would take years of inattention now for the drip to fill the bilge.

But, the knocking noise was getting worse and worse.  In addition, at higher RPMs, the dripless seal was throwing a lot of water around the bilge.  Not enough to worry, but an annoying amount.

This last week, Coastal Marine Engine, the folks who did the repower, said they had some new ideas.  Originally they had thought the bellows had a spring in it, and it turns out it doesn’t.  They found this while repowering another boat with a similar design.  They found that out of the water, as expected, the hull of the boat changes because there is no water pressure against it.  Once putting the boat into the water, the change in hull shape was enough to make the carbon disc on the boat change, and cause the shaft to be closer to the shaft log (hole through the boat that the shaft goes into) than they’d like.

They offered, free of charge, to come out and examine mine to make sure shaft alignment was correct, and to see if the dripless was being affected.

Sure enough, they found that the shaft alignment had changed, and adjusted things.  They also examined the dripless seal and found that the bellows were producing less tension than the other boat they worked on recently.  They adjusted things as best as they could, and advised me to have the dripless seal replaced during the next haul out.

In my simple testing at the dock, the knocking has almost completely disappeared.  The only time it shows up is when the motor is cold and at idle, where it’s vibrating the most, and it’s transmitting those vibrations down the shaft.  Once it’s warm, the knocking is gone.

The seal still leaks, and it still knocks a little, but it’s not so loud that people look at me when the motor is on.  I’ll have it replaced sometime this year, and I know that it will get much better as a result.

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